This invention generally relates to autostereoscopic display systems for viewing electronically generated images and more particularly relates to an apparatus and method for generating left- and right-eye images using a scanned linear micro-electromechanical modulator as its image source with a monocentric arrangement of optical components to provide a very wide field of view and large exit pupils.
The potential value of autostereoscopic display systems is widely appreciated particularly in entertainment and simulation fields. Autostereoscopic display systems include xe2x80x9cimmersionxe2x80x9d systems, intended to provide a realistic viewing experience for an observer by visually surrounding the observer with a 3-dimensional (3-D) image having a very wide field of view. As differentiated from the larger group of stereoscopic displays that include it, the autostereoscopic display is characterized by the absence of any requirement for a wearable item of any type, such as goggles, headgear, or special glasses, for example. That is, an autostereoscopic display attempts to provide xe2x80x9cnaturalxe2x80x9d viewing conditions for an observer.
In an article in SID 99 Digest, xe2x80x9cAutostereoscopic Properties of Spherical Panoramic Virtual Displays,xe2x80x9d G. J. Kintz discloses one approach to providing autostereoscopic display with a wide field of view. Using the Kintz approach, no glasses or headgear are required. However, the observer""s head must be positioned within a rapidly rotating spherical shell having arrays of LED emitters, imaged by a monocentric mirror, to form a collimated virtual image. While the Kintz design provides one solution for a truly autostereoscopic system having a wide field of view, this design has considerable drawbacks. Among the disadvantages of the Kintz design is the requirement that the observer""s head be in close proximity to a rapidly spinning surface. Such an approach requires measures to minimize the likelihood of accident and injury from contact with components on the spinning surface. Even with protective shielding, proximity to a rapidly moving surface could, at the least, cause the observer some apprehension. In addition, use of such a system imposes considerable constraints on head movement.
One class of autostereoscopic systems that operates by imaging the exit pupils of a pair of projectors onto the eyes of an observer is as outlined in an article by S. A. Benton, T. E. Slowe, A. B. Kropp, and S. L. Smith (xe2x80x9cMicropolarizer-Based Multiple-Viewer Autostereoscopic Display,xe2x80x9d in Stereoscopic Displays and Virtual Reality Systems VI, SPIE, January, 1999). Pupil imaging, as outlined by Benton in the above-mentioned article, can be implemented using large lenses or mirrors. An observer whose eyes are coincident with the imaged pupils can view a stereoscopic scene without crosstalk, without wearing eyewear of any kind.
It can be readily appreciated that the value and realistic quality of the viewing experience provided by an autostereoscopic display system using pupil imaging is enhanced by presenting the 3-D image with a wide field of view and large exit pupil. Such a system is most effective for immersive viewing functions if it allows an observer to be comfortably seated, without constraining head movement to within a tight tolerance and without requiring the observer to wear goggles or other device. For fully satisfactory 3-D viewing, such a system should provide separate, high-resolution images to right and left eyes. It can also be readily appreciated that such a system is most favorably designed for compactness, to create an illusion of depth and width of field, while occupying as little actual floor space and volume as is possible. For the most realistic viewing experience, the observer should be presented with a virtual image, disposed to appear a large distance away.
It is also known that conflict between depth cues associated with vergence and accommodation can adversely impact the viewing experience. Vergence refers to the degree at which the observer""s eyes must be crossed in order to fuse the separate images of an object within the field of view. Vergence decreases, then vanishes as viewed objects become more distant. Accommodation refers to the requirement that the eye lens of the observer change shape to maintain retinal focus for the object of interest. It is known that there can be a temporary degradation of the observer""s depth perception when the observer is exposed for a period of time to mismatched depth cues for vergence and accommodation. It is also known that this negative effect on depth perception can be mitigated when the accommodation cues correspond to distant image position.
An example of a conventional autostereoscopic display unit is disclosed in U.S. Pat. No. 5,671,992 (Richards), at which a seated observer experiences apparent 3-D visual effects created using images generated from separate projectors, one for each eye, and directed to the observer using an imaging system comprising a number of mirrors.
Conventional solutions for stereoscopic imaging have addressed some of the challenges noted above, but there is room for improvement. For example, some early stereoscopic systems employed special headwear, goggles, or eyeglasses to provide the 3-D viewing experience. As just one example of such a system, U.S. Pat. No. 6,034,717 (Dentinger et al.) discloses a projection display system requiring an observer to wear a set of passive polarizing glasses in order to selectively direct the appropriate image to each eye for creating a 3-D effect.
Certainly, there are some situations for which headgear of some kind can be considered appropriate for stereoscopic viewing, such as with simulation applications. For such an application, U.S. Pat. No. 5,572,229 (Fisher) discloses a projection display headgear that provides stereoscopic viewing with a wide field of view. However, where possible, there are advantages to providing autostereoscopic viewing, in which an observer is not required to wear any type of device, as was disclosed in the device of U.S. Pat. No. 5,671,992. It would also be advantageous to allow some degree of freedom for head movement. In contrast, U.S. Pat. No. 5,908,300 (Walker et al.) discloses a hang-gliding simulation system in which an observer""s head is maintained in a fixed position. While such a solution may be tolerable in the limited simulation environment disclosed in U.S. Pat. No. 5,908,300, and may simplify the overall optical design of an apparatus, constraint of head movement would be a disadvantage in an immersion system. Notably, the system disclosed in the Walker et al. patent employs a narrow viewing aperture, effectively limiting the field of view. Complex, conventional projection lenses, disposed in an off-axis orientation, are employed in the device disclosed in U.S. Pat. No. 5,908,300, with scaling used to obtain the desired output pupil size.
A number of systems have been developed to provide stereoscopic effects by presenting to the observer the combined image, through a beamsplitter, of two screens at two different distances from the observer, thereby creating the illusion of stereoscopic imaging, as is disclosed in U.S. Pat. No. 5,255,028 (Biles). However, this type of system is limited to small viewing angles and is, therefore, not suitable for providing an immersive viewing experience. In addition, images displayed using such a system are real images, presented at close proximity to the observer, and thus likely to introduce the vergence/accommodation problems noted above.
It is generally recognized that, in order to minimize vergence/accommodation effects, a 3-D viewing system should display its pair of stereoscopic images, whether real or virtual, at a relatively large distance from the observer. For real images, this means that a large display screen must be employed, preferably placed a good distance from the observer. For virtual images, however, a relatively small curved mirror can be used as is disclosed in U.S. Pat. No. 5,908,300 (Walker). The curved mirror acts as a collimator, providing a virtual image at a large distance from the observer. Another system for stereoscopic imaging is disclosed in xe2x80x9cMembrane Mirror Based Autostereoscopic Display for Tele-Operation and Telepresence Applications,xe2x80x9d in Stereoscopic Displays and Virtual Reality Systems VII, Proceedings of SPIE, Volume 3957 (McKay, Mair, Mason, Revie) which uses a stretchable membrane mirror. Although the apparatus disclosed in the McKay article provides a small exit pupil, it is likely that this pupil could be enlarged somewhat simply by scaling the projection optics. However, the apparatus disclosed in the McKay article has limited field of view, due to the use of conventional projection optics and due to dimensional constraints that limit membrane mirror curvature.
Curved mirrors have also been used to provide real images in stereoscopic systems, where the curved mirrors are not used as collimators. Such systems are disclosed in U.S. Pat. Nos. 4,623,223 (Kempf) and 4,799,763 (Davis et al.) for example. However, systems such as these are generally suitable where only a small field of view is needed.
Notably, existing solutions for stereoscopic projection project images onto a flat screen, even where that image is then reflected from a curved surface. This can result in undesirable distortion and other image aberration, constraining field of view and limiting image quality overall.
From an optical perspective, it can be seen that there would be advantages to autostereoscopic design using pupil imaging. A system designed for pupil imaging must provide separate images to the left and right pupils correspondingly and provide the most natural viewing conditions, eliminating any need for goggles or special headgear. In addition, it would be advantageous for such a system to provide the largest possible pupils to the observer, so as to allow some freedom of movement, and to provide an ultra-wide field of view. It is recognized in the optical arts that each of these requirements, by itself, can be difficult to achieve. An ideal autostereoscopic imaging system must meet the challenge for both requirements to provide a more fully satisfactory and realistic viewing experience. In addition, such a system must provide sufficient resolution for realistic imaging, with high brightness and contrast. Moreover, the physical constraints presented by the need for a system to have small footprint, and dimensional constraints for interocular separation must be considered, so that separate images directed to each eye can be advantageously spaced and correctly separated for viewing. It is instructive to note that interocular distance constraints limit the ability to achieve larger pupil diameter at a given ultrawide field by simply scaling the projection lens.
Monocentric imaging systems have been shown to provide significant advantages for high-resolution imaging of flat objects, such as is disclosed in U.S. Pat. No. 3,748,015 (Offner), which teaches an arrangement of spherical mirrors arranged with coincident centers of curvature in an imaging system designed for unit magnification. The monocentric arrangement disclosed in U.S. Pat. No. 3,748,015 minimizes a number of types of image aberration and is conceptually straightforward, allowing a simplified optical design for high-resolution catoptric imaging systems. A monocentric arrangement of mirrors and lenses is also known to provide advantages for telescopic systems having wide field of view, as is disclosed in U.S. Pat. No. 4,331,390 (Shafer). However, while the advantages of monocentric design for overall simplicity and for minimizing distortion and optical aberrations can be appreciated, such a design concept can be difficult to implement in an immersion system requiring wide field of view and large exit pupil with a reasonably small overall footprint. Moreover, a fully monocentric design would not meet the requirement for full stereoscopic imaging, requiring separate images for left and right pupils.
As is disclosed in U.S. Pat. No. 5,908,300, conventional wide-field projection lenses can be employed as projection lenses in a pupil-imaging autostereoscopic display. However, there are a number of disadvantages with conventional approaches. Wide-angle lens systems, capable of angular fields such as would be needed for effective immersion viewing, would be very complex and costly. Typical wide angle lenses for large-format cameras, such as the Biogon(trademark) lens manufactured by Carl-Zeiss-Stiftung in Jena, Germany for example, are capable of 75 degree angular fields. The Biogon lens consists of seven component lenses and is more than 80 mm in diameter, while only providing a pupil size of 10 mm. For larger pupil size, the lens needs to be scaled in size, however, the large diameter of such a lens body presents a significant design difficulty for an autostereoscopic immersion system, relative to the interocular distance at the viewing position. Costly cutting of lenses so that right- and left-eye assemblies could be disposed side-by-side, thereby achieving a pair of lens pupils spaced consistently with human interocular separation, presents difficult manufacturing problems. Interocular distance limitations constrain the spatial positioning of projection apparatus for each eye and preclude scaling of pupil size by simple scaling of the lens. Moreover, an effective immersion system most advantageously allows a very wide field of view, preferably well in excess of 90 degrees, and would provide large exit pupil diameters, preferably larger than 20 mm.
As an alternative for large field of view applications, ball lenses have been employed for specialized optical functions, particularly miniaturized ball lenses for use in fiber optics coupling and transmission applications, such as is disclosed in U.S. Pat. No. 5,940,564 (Jewell) which discloses advantageous use of a miniature ball lens within a coupling device. On a larger scale, ball lenses can be utilized within an astronomical tracking device, as is disclosed in U.S. Pat. No. 5,206,499 (Mantravadi et al.) In U.S. Pat. No. 5,206,499, the ball lens is employed because it allows a wide field of view, greater than 60 degrees, with minimal off-axis aberrations or distortions. In particular, the absence of a unique optical axis is used advantageously, so that every principal ray that passes through the ball lens can be considered to define its own optical axis. Because of its low illumination falloff relative to angular changes of incident light, a single ball lens is favorably used to direct light from space to a plurality of sensors in this application. Notably, photosensors at the output of the ball lens are disposed along a curved focal plane.
The benefits of a spherical or ball lens for wide angle imaging are also utilized in an apparatus for determining space-craft attitude, as is disclosed in U.S. Pat. No. 5,319,968 (Billing-Ross et al.) Here, an array of mirrors direct light rays through a ball lens. The shape of this lens is advantageous since beams which pass through the lens are at normal incidence to the image surface. The light rays are thus refracted toward the center of the lens, resulting in an imaging system having a wide field of view.
Another specialized use of ball lens characteristics is disclosed in U.S. Pat. No. 4,854,688 (Hayford et al.) In the optical arrangement of the Hayford et al. patent, directed to the transmission of a CRT-generated 2-dimensional image along a non-linear path, such as attached to headgear for a pilot, a ball lens directs a collimated input image, optically at infinity, for a pilot""s view.
Another use for wide-angle viewing capabilities of a ball lens is disclosed in U.S. Pat. No. 4,124,978 (Thompson), which teaches use of a ball lens as part of an objective lens in binocular optics for night viewing.
With U.S. Pat. Nos. 4,124,978 and 4,854,688 described above disclose use of a ball lens in image projection, there are suggestions of the overall capability of the ball lens to provide, in conjunction with support optics, wide field of view imaging. However, there are substantial problems that must be overcome in order to make effective use of such devices for immersive imaging applications, particularly where an image is electronically processed to be projected. For example, conventional electronic image presentation techniques, using devices such as spatial light modulators, provide an image on a flat surface. Ball lens performance with flat field imaging would be extremely poor.
There are also other basic optical limitations for immersion systems that must be addressed with any type of optical projection that provides a wide field of view. An important limitation is imposed by the LaGrange invariant. Any imaging system conforms to the LaGrange invariant, whereby the product of pupil size and semi-field angle is equal to the product of the image size and the numerical aperture and is an invariant for the optical system. This can be a limitation when using, as an image generator, a relatively small spatial light modulator or similar pixel array which can operate over a relatively small numerical aperture since the Lagrange value associated with the device is small. A monocentric imaging system, however, providing a large field of view with a large pupil size (that is, a large numerical aperture), inherently has a large LaGrange value. Thus, when this monocentric imaging system is used with a spatial light modulator having a small LaGrange value, either the field or the aperture of the imaging system, or both, will be underfilled due to such a mismatch of Lagrange values. For a detailed description of the Lagrange invariant, reference is made to Modern Optical Engineering, The Design of Optical Systems by Warren J. Smith, published by McGraw-Hill, Inc., pages 42-45.
Commonly-assigned U.S. Pat. No. 6,416,181 (Kessler et al.) and copending U.S. patent application Ser. No. 09/854,699 take advantage of capabilities for wide field of view projection using a ball lens in an autostereoscopic imaging system. In these copending applications, the source image that is provided to the projecting ball lens for each eye is presented as a complete two-dimensional image. The image source disclosed in each of these applications is a two-dimensional array, such as a liquid crystal display (LCD), a digital micromirror device (DMD), or similar device. The image source could alternately be a cathode ray tube (CRT) which, even though generated by a scanned electron beam, presents a complete two-dimensional image to ball lens projection optics.
There are some inherent limitations in providing a complete two-dimensional image. Ideally, a curved image field is preferred, with the center of curvature of this field coincident with the center of the ball lens, since this arrangement minimizes field aberrations. However, providing a curved image field requires either curving the image source itself or providing an additional faceplate or special relay optics in the imaging path. Curving a two-dimensional image array to obtain or approximate spherical curvature of the image source would be difficult and costly. Employing a faceplate or special relay optics with a planar image array has disadvantages including added cost and overall loss of brightness. Maintaining sufficient brightness for projection is a concern when using small two-dimensioned arrays, since this can be difficult to achieve without special design techniques and higher-cost components. Thus, it can be appreciated that there can be improvements to overall cost of system optics for generating and projecting images for stereoscopic viewing.
Ball lenses and ball lens segments have been used as scanning components in sensor applications for wide field-of-view optical scanning. U.S. Pat. No. 6,233,100 (Chen et al.) discloses a concentric sensor scanning system that employs a rotatable scanning ball lens segment with one or more reflective facets. In the system disclosed in U.S. Pat. No. 6,233,100, rotation of a ball lens or ball lens segment directs incoming radiation onto a concentric row of sensors. However, existing projection systems designs have utilized more conventional projector optics components and, in doing this, have overlooked possible deployment of ball lenses or ball lens segments as scanning components for projecting light in a scanned fashion in order to produce an image.
There are a number of techniques used to form a two-dimensional image by scanning, either with either a point source, such as a conventional CRT electron beam, or with a linear source. U.S. Pat. No. 6,511,182 (Agostinelli et al.) discloses the use of a number of types of linear sources with a scanning system. Among solutions proposed in U.S. Pat. No. 6,511,182 are LED arrays and resonant fiber optic scanners.
Microelectromechanical devices have been developed as spatial light modulators in a variety of applications, including optical processing, printing, optical data storage, spectroscopy, and display. Microelectromechanical modulators include devices such as grating light valves (GLVs), developed by Silicon Light Machines, Sunnyvale, Calif. and described in U.S. Pat. No. 5,311,360 (Bloom et al.) and electromechanical conformal grating devices as disclosed in U.S. Pat. No. 6,307,663 (Kowarz). These modulators produce spatial variations in phase and amplitude of an incident light beam using arrays of individually addressable devices that are arranged as a periodic sequence of reflective elements forming electromechanical phase gratings. Such microelectromechanical grating devices are of particular interest as linear modulators because they provide sufficient speed for two-dimensional displays and have very good contrast and optical efficiency. At the same time, these devices are mechanically compact and rugged and can be produced at relatively low cost. However, microelectromechanical modulators have been largely overlooked as suitable components for immersive optics applications providing wide field of view. With the advent of low-cost laser light sources, however, there is opportunity for exploiting light-efficient alternatives such as microelectromechanical modulators in intermediate- and large-size immersion display systems. It is necessary, however, to couple this type of light modulation solution with an image projection system that is capable of providing the wide field of view needed for effective immersion optics.
Thus, it can be seen that, while there are some conventional approaches that meet some of the requirements for stereoscopic imaging, there is a need for an improved autostereoscopic imaging solution for viewing electronically generated images, where the solution provides a structurally simple apparatus, minimizes aberrations and image distortion, and meets demanding requirements for wide field of view, large pupil size, high brightness, and lowered cost.
Briefly, according to one aspect of the present invention, an autostereoscopic optical apparatus for viewing a stereoscopic virtual image comprises an array of image pixels, the stereoscopic virtual image comprising a left image to be viewed by an observer at a left viewing pupil and a right image to be viewed by the observer at a right viewing pupil, the apparatus comprising:
(a) an image generation system for forming a left two-dimensional intermediate image and for forming a right two-dimensional intermediate image, the image generation system comprising:
(a1) a linear image modulator for modulating an incident light beam from a light source to form a modulated light beam;
(a2) a first beamsplitter for splitting the modulated light beam into a left image beam and a right image beam, the left image beam directed to a left diffusive surface for forming a left line of source pixels and the right image beam directed to a right diffusive surface for forming a right line of source pixels;
(a3) a left scanning ball lens assembly for projecting the left line of source pixels to form a left intermediate line image and a right scanning ball lens assembly for projecting the right line of source pixels to form a right intermediate line image, each scanning ball lens assembly comprising:
(a3a) at least one reflective surface for reflecting light from the left or right line of source pixels to the left or right intermediate line image, respectively;
(a3b) a ball lens segment having a scanning ball lens pupil, the ball lens segment having a center of curvature on the at least one reflective surface;
each left and right scanning ball lens assembly rotating about an axis and forming a series of adjacent the intermediate line images in order to sequentially form the left and right two-dimensional intermediate image thereby;
(b) a curved mirror having a center of curvature placed substantially optically midway between the left scanning ball lens assembly and the right scanning ball lens assembly;
(c) a second beamsplitter disposed to form the left two-dimensional intermediate image near a front focal surface of the curved mirror and to form the right two-dimensional intermediate image near the front focal surface of the curved mirror; and
the curved mirror forming the virtual stereoscopic image of the left and right two-dimensional intermediate images and, through the second beamsplitter, forming a real image of the left scanning ball lens pupil at the left viewing pupil and a real image of the right scanning ball lens pupil at the right viewing pupil.
A feature of the present invention is the use of a monocentric arrangement of optical components, thus simplifying design, minimizing aberrations and providing a wide field of view with large exit pupils.
A further feature of the present invention is the use of an electromechanical grating device for forming a linear image, scanned by a ball lens segment having a reflective surface in order to provide a scanned, two-dimensional intermediate image.
A further feature of the present invention is that it allows a number of configurations, including configurations that minimize the number of optical components required, even including configurations that eliminate the need for a beamsplitter.
It is an advantage of the present invention is that it eliminates the need for a higher cost two-dimensional surface as image source, replacing this with a lower cost electromechanical grating device.
It is a further advantage of the present invention that it allows use of inexpensive, bright light sources for generating an intermediate image for projection. The high spectral purity of laser sources helps to maximize the achievable color gamut for a display apparatus.
It is a further advantage of the present invention that it provides a compact arrangement of optical components, capable of being packaged in a display system having a small footprint.
It is a further advantage of the present invention that it allows high-resolution stereoscopic electronic imaging with high brightness and high contrast, with a very wide field of view. The present invention provides a system that is very light-efficient, capable of providing high brightness levels for projection.
It is a further advantage of the present invention that it provides a solution for wide field stereoscopic projection that is inexpensive when compared with the cost of conventional projection lens systems.
It is a further advantage of the present invention that it provides stereoscopic viewing without requiring an observer to wear goggles or other device.
It is yet a further advantage of the present invention that it provides an exit pupil of sufficient size for non-critical alignment of an observer in relation to the display.
These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.